This invention relates generally to RF power amplifiers, and more particularly the invention relates to an improved DC voltage feed network to a power amplifier thereby permitting improved wideband operation.
DC electrical power for an RF power amplifier is normally connected to a bipolar transistor collector or to an FET transistor drain through a coil or choke to minimize attenuation of the amplified carrier signal and loss of power in undesired heating of the choke. Referring to FIG. 1, the conventional circuit includes an RF bypass capacitor in the DC power source with the power source connected to the power amplifier active device through the choke which is represented by an inductance, L.sub.1, and a small parasitic resistance R.sub.1. An output matching network connects the power amplifier to the load, R.sub.L.
The choke impedance L.sub.1 is selected to be approximately 10 times or greater than the transformed value of the load impedance, R.sub.L, at the point X, thereby eliminating any loading effect on the RF signal. The characteristics of any RF choke will vary with frequency, from characteristics resembling those of a parallel resonant circuit of high impedance, to those of a series resonant circuit where the impedance is lowest. In between these extremes the choke will show varying amounts of inductive or capacitive reactance and, the choke will also have a small amount of parasitic resistance, R.sub.S. The idealized signal attenuation of the bias network shown in FIG. 1 is shown in FIG. 2 where f.sub.C is a cutoff frequency of the inductive network.
In a parallel feed circuit, the choke is shunted across the load and is subject to the full output RF voltage. If the choke does not present a relatively high impedance, sufficient power will be absorbed by the choke parasitic resistance to cause undesired heating and power loss. To avoid this, the choke must have a sufficiently high reactance to be effective at the lowest operating frequency and yet have no series resonances across the operating frequency band. An appropriate value capacitor, C.sub.1, is used to shunt the DC input and decouple residual RF from the DC source.
Power amplifiers are used where the efficiency and output power of an amplifying circuit are important considerations. The various types of power amplifiers are identified by their classes of operation, i.e. classes A, B, C, D, E, F, G, H, and S. Except for class A, all of these amplifier types are easily differentiated from small signal amplifiers by their circuit configurations, methods of operations, or both. There is no sharp dividing line between small signal and class A power amplifiers, the choice of terms depends on the intent of the designer. Class A solid state power amplifiers are capable of providing highly linear amplification. However, they are considered cost prohibitive for high power transmitters (e.g. greater than 100 watts) because of relatively low power efficiency.
Class B or AB solid state power amplifiers do not have the high dynamic range linearity that class A solid state power amplifiers have. However, the power that the class B and AB amplifiers can provide is typically 3-5 times greater with far superior efficiency. A solid state device in a class A power amplifier is always biased fully on, which means the device will pull the same current through the bias network whether it is amplifying a large signal or a small signal. The transistor in a class B or AB power amplifier is biased so that it is only slightly on, it will pull from the bias network current in proportion to the signal driving the amplifier. When the amplifier is transmitting the highest average power, it will pull the required current needed to get full power out. However, when transmitting a signal has less average power, a significantly less amount of current will be drawn.
Class B and AB power amplifiers built with power semiconductor devices and operating with the classical bias scheme shown in FIG. 1 exhibit non-linearities that should be reduced if the output is to reproduce a multi-tone input signal with reasonable fidelity. When more than one tone are amplified simultaneously (e.g. signals F.sub.A and F.sub.B) a frequency difference signal is generated (F.sub.B-A). The bias network must not significantly attenuate either the DC current or the current at the difference frequency (F.sub.B-A) at the output point X in FIG. 1. That is, the choke should filter out the difference frequency F.sub.B-A. If a difference signal F.sub.B-A is attenuated by the choke, undesired distortions will be generated at the output of the amplifier.
The classical filter response as shown in FIG. 2 does not provide significant attenuation of the difference signal for a multi-tone input so long as the signals are close in frequency, as shown in FIG. 3A. However, in the case of Cellular Phone Base station amplifier signals, the frequency separation between F.sub.A and F.sub.B can be as much as 30 MHz, and therefore the difference signal may occur at a frequency that is high enough to fall in the attenuation band of the D.C. feed network as shown in FIG. 3B.